1. Field of the Invention
This invention relates to acoustic vibration sensing apparatus having a light transmitting fiber.
2. Description of the Prior Art
Many sensors for acoustic vibrations and other fluid pressure effects are well known and have various deficiencies such as low sensitivity, response that varies undesirably with direction, high cost, limited maximum pressure, and errors due to temperature and static pressure. The flexure or other strain of a primary elastic element subjected to a pressure difference is a well-known basis for acoustic and other pressure measurements, the amount of flexure typically being determined directly, or by piezoelectric, capacitive, or resistive changes in secondary elements associated with the primary element.
It is well-known to generate acoustic vibrations by an elastic shell which is generally ellipsoidal and has an internal piezoelectric disk or rod extending across the equator of the shell and coupled thereto so that, when electric signals of the usual frequencies employed with hydrophones are applied to the piezoelectric element, the resulting dimensional changes thereof cause the shell to oscillate and project corresponding acoustic vibrations in a surrounding fluid. Such a prior art projecting device, which has been termed a "flextensional transducer", is, therefore, an impedance transducer for converting vibrations of a piezoelectric element to fluid vibrations. Insofar as known to the present applicants, the use and sensitivity of a similar shell as the primary element of a receptor for acoustic vibrations has never been previously considered. In such prior art projecting transducers, the ellipsoidal shell is forced to oscillate in an equatorial plane or, in many cases, only along one axis of such a plane, so that the ratio of the lengths of the minor and major axes of the shell is not significant and has not been investigated. Also, in such a prior art transducer the forced shell oscillations are not significantly affected by temperature as is typically the case with such secondary elements used in an acoustic vibration sensor.
Interferometric measurements of strain can provide great resolution and, when carried out with an optical fiber, can provide a simple and rugged sensor which requires low power, is immune to many forms of interference, and is adapted to remote sensing of pressure variations and to high data rates. However, optical fibers are relatively insensitive per unit length when used directly for measurement of pressure variations and are subject to errors due to static pressure, temperature, and the like. Increase in leg length to provide greater sensitivity typically increases such errors proportionately,and interferometers having an optical fiber leg are particularly subject to error from temperature caused variations in the length of the leg. It is known to minimize these errors by a "push-pull" arrangement of a pair of interferometer optical fiber legs where a change in a measured variable shortens one leg and lengthens the other while both legs change length together with variations in temperature and static pressure. However to be effective, this interferometric rejection of common mode errors requires that both legs be subject to the same conditions.
It is known to provide optical fiber interferometric measurements of pressure and variations thereof by arranging a resilient cylinder, which is wound with an optical fiber, for compression resulting from strain of a primary element so that lateral expansion of the cylinder lengthens the fiber. This optical fiber arrangement for pressure measurement is effective and is adapted to push-pull operation by using two cylinders and winding an optical fiber leg under tension on each cylinder, but is somewhat limited in sensitivity. Also, typical existing such arrangements are directional, are relatively complex in construction since the fibers and cylinders are disposed within a primary sensing element, and reject common mode errors imperfectly since the push-pull optical fiber legs are separated to some extent spatially and thermally.